Swirlers can be provided for mixing devices in various technical applications. Optimization of swirlers aims to reduce energy to obtain a specified degree of homogeneity. In continuous flow mixing, a pressure drop over a mixing device is a measure of energy. Further, the time and space to obtain the specified degree of homogeneity are useful parameters when evaluating mixing devices or mixing elements. Swirlers can be used for mixing of two continuous fluid streams.
High volume flows of gas can be, for example, mixed at an outlet of a turbofan engine, where hot exhaust gases of a core engine mix with relatively cold and slower bypass air. In order to reduce sound emissions caused by these different flows lobe mixers are disclosed in U.S. Pat. No. 4,401,269.
One application for mixing of continuous flow streams is the mixing of a fuel with an oxidizing fluid, for example air, in a burner for premixed combustion in a subsequent combustion chamber. In modern gas turbines, good mixing of fuel and combustion air is desirable for complete combustion with low emissions.
To achieve high efficiency, a high turbine inlet temperature is useful in standard gas turbines. As a result, there can arise high NOx emission levels and higher life cycle costs. These can be mitigated with a sequential combustion cycle, wherein the compressor delivers nearly double the pressure ratio of a known one. The main flow passes the first combustion chamber (e.g. using a burner of the general type as disclosed in EP 1 257 809 or as in U.S. Pat. No. 4,932,861, also called EV combustor, where the EV stands for EnVironmental), wherein a part of the fuel is combusted. After expanding at the high-pressure turbine stage, the remaining fuel is added and combusted (e.g. using a burner of the type as disclosed in U.S. Pat. No. 5,431,018 or U.S. Pat. No. 5,626,017 or in U.S. Patent Application Publication No. 2002/0187448, also called SEV combustor, where the S stands for sequential). Both combustors contain premixing burners, as low NOx emissions require high mixing quality of the fuel and the oxidizer.
Because the second combustor is fed by the expanded exhaust gas of the first combustor, the operating conditions can allow self-ignition (spontaneous ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein should not exceed the auto ignition delay time. This criterion can ensure flame-free zones inside the burner but can pose challenges in obtaining appropriate distribution of the fuel across the burner exit area.
SEV-burners can be designed for operation on natural gas and oil. Therefore, the momentum flux of the fuel can be adjusted relative to the momentum flux of the main flow so as to penetrate in to the vortices. This can be done using air from the last compressor stage (high-pressure carrier air). The high-pressure carrier air bypasses the high-pressure turbine. The subsequent mixing of the fuel and the oxidizer at the exit of the mixing zone is sufficient to allow low NOx emissions (mixing quality) and avoid flashback (residence time), which can be caused by auto ignition of the fuel air mixture in the mixing zone.